Thermal growth can damage your equipment and processes, not to mention your bottom line. Monitoring positional changes it causes is a good way to keep this pesky phenomenon from getting out of hand.
By Pedro Casanova, LUDECA, Inc.
LUDECA was contacted by a customer about a Cooper Turbo Compressor that was exhibiting high levels of vibration—the kind that could impact the unit’s long-term operation and reliability. Fortunately, the customer owns a ROTALIGN ULTRA laser shaft-alignment system (Fig. 1, above) equipped with the optional LIVE TREND module that can monitor positional changes due to thermal growth over time. That capability offers great value for operations whose critical equipment systems are subject to thermal growth situations.
Thermal growth can be an expensive proposition for a plant. As machines heat up, thermal expansion causes changes in the alignment condition, which can lead to higher vibration and temperatures, along with higher power consumption and a consequent increase in maintenance costs.
Fig. 2. The LIVE TREND monitor bolted into place
The customer’s turbo compressor is driven by a motor rated as 1750 hp at 3550 RPM. The alignment targets that the customer was using were called into question when vibration continued even after performing a successful shaft alignment with the ROTALIGN ULTRA. To verify—and possibly redefine—the targets to be used for future alignments, the LIVE TREND monitoring system was set up on the machine, with the brackets firmly bolted into place as shown in Fig. 2. (Although not employed in the customer’s application, simple repair putty can be used, which would reduce the time needed to get the LIVE TREND module into place.)
After collecting data for about three hours, the original targets for Vertical Offset (VO1) of +10.0 mils and Vertical Angularity (VA1) of +4.0 mils at an 8.5-inch diameter coupling were confirmed to be inaccurate. Thus, a new VO target of -6.4 mils and VA target of +6.6 mils per 8.5 inches were established. Figure 3 shows the final growth data. (NOTE: Since the monitoring was conducted from “Cold to Hot,” the Cold Alignment Targets will be the same values, but with opposite signs.)
Fig. 3. Final growth data per LIVE TREND
Calculating the costs of inaccuracy
The original VO target was so inaccurate that it was no wonder the machine was vibrating severely at operating temperature. The motor had been set 10.0 mils high (VOt = +10 mils) instead of 6.4 mils low, as the LIVE TREND data suggested it should be. This state of affairs caused the machines to be misaligned by as much as 16.4 mils. Although the original VA target error of 2.6 mils was less severe, it still needed to be addressed.
It was decided that the new targets would be used to align the machines at the next opportunity.
Calculating the benefits of improved trending
The next scheduled downtime for the problem unit occurred a month later. Once the equipment was realigned to the new targets, the team hoped to see a reduction in the damaging vibration levels they had been experiencing. When the machinery was started, it stabilized at normal operating temperature. Incredibly, the vibration levels (as shown in the before-and-after data of Table I) were measured at near zero. MT
Pedro Casanova is Senior Alignment Application Engineer with LUDECA, Inc. Telephone: (305) 591-8935; email: firstname.lastname@example.org.